The invention concerns an electronic angle coder of the type comprising a rotating disk carrying a series of radial marks capable of being registered by detectors placed in front of the path of rotation of the marks engraved.
At least one reference detector is provided, together with at least one measuring detector, the latter being mobile in rotation around the axis of rotation of the disk. Such a coder may find an application for example in sighting telescopes or theodolites. The angle of rotation of the axis of the mobile measuring detector is to be measured with respect to a reference position represented by the position of the reference detector.
Double measurements are performed for the purpose of obtaining approximate measures and fine measures.
The approximate measure is obtained by counting the number of engraved marks registered by one of the detectors between the passage in front of the detector of a mark or a characteristic reference point of the rotating disc and the passage of the same mark or an associated mark in front of another detector. Such an approximate measure yields an angular value evaluated in the number of graduations of the disk located at any moment between the reference detector and the measuring detector.
The fine measurement consists of the additional determination of the exact supplemental angle of displacement between the two detectors when the total displacement does not correspond exactly to a whole number of intervals between succesive marks of the rotating disk. This fine measurement is performed by the determination by means of a numerical phase meter of the duration separating the passage of any radial mark in front of one of the detectors and the passage of a mark in front of the other detector after the first passage mentioned herein (preferably immediately after).
To improve the precision of the fine measurement, in view of possible errors in the regularity of the engraving of the marks of the rotating disk, the fine measure is taken for each passage of a mark in front of the reference detector, for a large number of consecutive marks (preferably over a complete rotation or several complete rotations of the disk), followed by the calculation of the average of the fine measures registered thus to derive a better approximation of the total fine measure; the calculation shows that engraving errors are practically eliminated if the average is calculated for a number of engraved consecutive marks as close as possible to the number corresponding to a complete rotation to a complete rotation (or several complete rotations).
A problem arises in connection with this averaging of the fine measures of phase displacement, when said phase displacement is close to 0 or to 2.pi., i.e. when the two detectors (the reference and the measuring detectors) observe nearly simultaneousy in front of them a mark of the rotating disk and therefore their angular distance is a practically exact multiple of the angular interval between the two marks.
In effect, in such a case, where there are fluctuations of the spacing of the engraved lines, where the spacing is not absolutely perfect, there is a risk that the phase meter will measure a small positive angle for a first measure and an angle slightly less than 2.pi. for the following measure, with the average of the values not yielding a value close to zero, as they should. For n number of measures, if the fluctuations cause the measure to pass k times to the other side of zero, the average fine measure calculated will be close to 2k.pi./n instead of zero, which is inadmissible.
This ambiguity of the values close to zero results from the fact that one phase measurement is by a 2.pi. modulus and that a negative phase displacement close to zero will yield a positive indication close to 2.pi..